Makes 6 to 8 servings
Start to Finish: 45 minutes
Directions
1. Preheat the oven to 375°F. Grease a 9-by-13-inch casserole dish with 1 tablespoon of the butter.
2. In a large pot, heat the remaining 4 tablespoons butter over medium heat. Add the onion and sauté until tender and translucent, 4 to 5 minutes. Add the garlic and cook until fragrant, 1 minute more.
3. Stir in the mustard. Add the half-and-half and bring to a simmer. Add the potatoes and simmer for 4 to 5 minutes. Season with the salt, pepper and nutmeg.
4. Pour the potato mixture into the prepared casserole dish and spread into an even layer. Top with the cheese in an even layer. (The dish can be stored, covered, in the refrigerator at this point if you want to prepare it ahead.)
5. Bake until the cheese is golden, the sauce is bubbly and the potatoes are easily pierced with a fork, 25 to 30 minutes (add 5 to 7 minutes if baking after storing in the refrigerator). Let cool 5 minutes before serving warm.